(1) Field of the Invention
The present invention relates to screw compressor and more specifically to a screw compressor for compressing gas in the state where liquid is mixed to the gas.
(2) Description of the Related Art
A general screw compressor in the related art will be explained with reference to FIG. 5 and FIG. 6. FIG. 5 is a cross-sectional view of a delivery end in the state immediately before delivery completion of a general screw compressor in the related art. FIG. 6 is an enlarged cross-sectional view of the delivery end in the state at the moment of delivery completion in FIG. 5.
In the screw compressor, a pair of female rotor 1 and a male rotor 2 are accommodated within a bore 21 of a casing 20 indicated with a broken line to respectively rotate in the arrow mark direction, and are meshed with each other as shown in FIG. 5. With progress in rotation of both rotors 1 and 2, gas within a groove operating as a working chamber is compressed and is delivered to a delivery chamber (not shown) through a delivery port 3.
At the meshed area of both rotors 1 and 2, a working chamber 7 and a working chamber 8 are formed and respectively include a contact point 4 and a contact point 5, a contact point 4 and a contact point 6 at both ends thereof.
One working chamber 7 is formed in the adequate groove shape while those volumes are expanded in association with rotation of the rotors 1 and 2. This working chamber 7 is communicated with a suction port (not shown) at the other ends of the rotors 1, 2.
The other working chamber 8 is formed in the adequate groove shape while gradually reducing in the volume. This working chamber 8 becomes a closed space for external side, except for the delivery port 3, immediately before the delivery completion. Liquid is poured to the working chamber 8 for cooling the gas in the compression process and hermetically sealing a clearance of the working chamber that will resulting in internal leak, and the gas mixed with the liquid is compressed in the working chamber 8. In the delivery process, since the gas having the density smaller than that of the liquid is delivered previously, the working chamber 8 is filled with the liquid immediately before the delivery completion, and the gas is almost ruled out.
When the rotors 1, 2 further rotate, the working chamber 8 changes into a closed working chamber 9 because it is isolated from the delivery port 3 as shown in FIG. 6. Even when the volume of the closed working chamber 9 is further reduced after the rotors 1, 2 further rotate, an exit of liquid is not provided within the interior. Therefore, this is the possibility that not only pressure within the closed working chamber 9 is likely to rise rapidly and vibration and noise are likely to be generated, but also damage of rotor and shortening in operation life of a bearing are likely to be caused.
Therefore, Japanese Examined Patent Application Publication No. S62-358 (Patent document 1) discloses another screw compressor. This screw compressor eliminates confinement of liquid and reduces vibration and noise level by providing a recessed part on an internal wall surface opposing to a rotor delivery end of a casing, forming an area of contour of the recessed part in the shape substantially conforming to a shape of a preceding flank of a groove forming a closed working chamber of a female rotor when the working chamber is isolated from the delivery port to form the closed working chamber, and by communicating the closed working chamber and the recessed part after the working chamber is isolated from the delivery port and changes into the closed working chamber.
[Patent Document 1] Japanese Examined Patent
In the screw compressor of the patent document 1, the closed working chamber and the recessed part are communicated with each other after the working chamber is isolated from the delivery port and changes into the closed working area, not considering that an internal pressure of the working chamber becomes very high immediately before the working chamber is isolated from the delivery port.
That is, since the gas having the density that is smaller than that of the liquid is delivered previously in the delivery process, the working chamber immediately before isolating from the delivery port is filled with the liquid and therefore the gas is almost ruled out. Accordingly, since the liquid is delivered through an extremely narrow communicating region between the delivery port having a reduced area and the working chamber immediately before the working chamber is isolated from the delivery port, it has become obvious that several problems are generated with inclusion of sharp increment in internal pressure of the working chamber, intermittent increase in torque for driving the rotors, and resultant increase in power consumption, vibration, and noise.
Here, the screw compressor in the patent document 1 has been limited only to a screw compressor where the closed working chamber is formed because the working chamber is isolated from the delivery port immediately before the delivery completion. Therefore, such screw compressor has a problem that it cannot be applied to the screw compressor where a volume of the working chamber substantially becomes zero at the moment when the working chamber is isolated from the delivery port.